I happened to hit this trying to cover other cases around valid vs missing
entries. I have no idea if this is something that could occur more naturally
(similar to how a missing file node in hgtagsfnodes1 can occur after a strip).
There is a test just above this added in f5a7cf0adb12 mentioning it "overwrites
the junk", though that tests truncation instead of actual garbage.
But since this is just a cache, it probably shouldn't abort with a cryptic
message like this. The two options I see both have downsides- either rebuild
the cache (and potentially take a long time), or hint to the user to run a debug